CLOVER MANAGEMENT TO PROVIDE OPTIMUM NITROGEN AND SOIL-WATER CONSERVATION

Legumes are commonly used to provide nitrogen (N) for succeeding crops , but the net gain in N to the system is rarely measured. This study d eveloped a N budget for a winter legume production system to estimate sources, sinks, and net system gain in N. Crimson clover (Trifolium in carnatum L.) reseeded from the preceding year and was killed with herb icide the following spring. A forage sorghum crop (Sorghum vulgare Per s.) was planted and harvested twice during the summer. Subplots were s ampled at frequent intervals throughout the growing seasons for measur ements of plant biomass and N content (leaves, stems, seeds, roots, an d litter) and soil mineral N. During the clover growing season, soil N mineralization and leaching rates were estimated with an insitu chamb er technique, and aerial ammonia (NH3) transport measurements were mad e biweekly by micrometeorological techniques. Nitrogen in the clover c rop increased until anthesis, and then declined slightly prior to desi ccation with herbicides. Total N accumulated in the clover at desiccat ion was 323 kg N ha(-1) (28 In leaves, 81 in stems, 40 in seeds, 44 in surface-layer roots, and 130 in dead leaves and litter). Aerial NH3 a bsorption by the clover was small during the growing season (0.18 kg N ha(-1)) but NH3 loss from the killed clover was also minimal (0.25 kg N ha(-1)) and occurred during the period shortly after herbicide appl ication. The summer sorghum crop took up 454 kg N ha(-1) over the cour se of two cuttings from the soil mineral and mineralized organic N res ource plus atmospheric NH3 and other atmospheric N inputs such as wet and dry deposition. Total N accumulated by the clover from N fixation, soil mineral N uptake, and NH3 absorption provided the equivalent of 70% of N removed by the sorghum crop.